Powdered materials can be compacted and formed into shapes of extended length by extrusion. Generally speaking, a progressively threaded extrusion screw is rotated at a preselected rate to convey a feed material into an enclosed section to an extrusion die opening. Frictional forces through the die opening resist the forward conveying action of the progressive screw and compact the feed material as it passes thru the die opening.
Inorganic materials, such as ceramics, can benefit from the heat and friction of the passage thru the die opening. Such ceramic extrudates are effectively dewatered and compacted sufficiently to form a wet “green” extrudate of sufficient integrity to withstand handling and more complete drying. Organic materials, however, can be severely degraded by the high temperatures encountered when passing thru an extrusion die. Such materials are either not extruded or require such slow extrusion screw speeds that the manufacturing process is limited in its production rate.
Many active ingredients are in the form of powders, free flowing crystals, or other types of finely divided solids. Such active ingredients are, for many applications, most useful in the form of larger aggregates or granules. A variety of granulation methods can be used to process such materials.
Granulation of heat sensitive active ingredients has traditionally excluded the use of extrusion and other high shear types of granulation methods due to high temperature rises at the die. It is not uncommon for some extruders operating at normal, commercial extrusion rates to expose the extruded material to temperature increases of 25° to 100° C. This is acceptable for extruded ingredients that are not degraded or otherwise harmed by such temperatures but can be disastrous for organic or biologic active ingredients that cannot withstand such temperatures. Thus, the controls and manufacturing efficiencies that attend extrusion techniques. It would be desirable to have an extrusion process for heat sensitive active ingredients that did not expose the extruded active ingredient to deleterious temperature increases at the extrusion die but which would form adequately dense extrudates at a commercially acceptable rate.
High temperatures have also prevented many useful pest control agents, herbicides, and plant growth regulating agents from being offered as a granule. Such materials are often chemical compounds that contain heat sensitive linkages that would be harmed by exposure to elevated temperatures.
N-hydrocarboyl phosphoroamidothioates and phosphoroamidodithioates (referred to herein as “phosphoroamido(di)thioates”) are classes of particularly heat sensitive compounds that are used as systemic insecticides in a variety of environments. One of the most commercially important compounds within this class is acephate. Acephate and related compounds are described in U.S. Pat. Nos. 3,716,600, 3,845,172 and 3,914,417.
Orthene® is a commercial form of acephate that is produced as a technical grade chemical of about 97 to 99.5% purity. It is available as a liquid and fine powder. The formation of a pellet or granular form with commercially acceptable properties has, thus far, eluded the art despite significant efforts.
Chevron and Valent have received a number of patents for processes to manufacture pelleted or granular acephate. Chan et al. U.S. Pat. No. 5,075,058 describes phosphoroamido(di)thioate pellets with a second active ingredient (insecticide, fungicide, herbicide, or fertilizer), a surfactant that is used to encapsulate the phosphoroamido(di)thioate active, an anhydrous magnesium sulfate as a dehydrating agent to absorb moisture and prevent hydrolysis of the phosphoroamido(di)thioate, a deodorant, and an anti-foaming agent. The mix is extruded thru a die at 91–100° F. and dried.
Chan et al. U.S. Pat. No. 5,100,667 describes a solvent-free method for making phosphoroamido(di)thioate pellets that relies on a dry mix with a solid surfactant to provide structural integrity. The example shows the use of ammonium sulfate in addition to the phosphoroamido(di)thioate and surfactant.
Chan et al. U.S. Pat. No. 5,464,623 teaches two processes to pelletize phosphoroamido(di)thioates. One uses a solvent for the technical to make a pourable or extrudable mixture. The list of preferred solvents include hexane, carbon tetrachloride, toluene, isopropanol, ethanol, chloroform, methanol, and methylene chloride. The other process avoids use of a solvent and melts the technical at about 90° C. for subsequent molding or spraying into droplets.
Cummings U.S. Pat. No. 5,298,501 describes the use of 83–98 wt % ammonium sulfate for providing integrity to granules containing 2–17 wt % of a phosphoroamido(di)thioate.
Cummings U.S. Pat. No. 5,352,674 discloses a formulation containing a phosphoroamido(di)thioate, an optional second active ingredient (e.g., a fungicide), at least 75 wt % of ammonium sulfate, 0.2–5 wt % of a surfactant, 0.05–2 wt % of a deodorant, and 1–5 wt % of granular processing aids that are selected from a lubricant (Mg stearate, Ca stearate, Zn stearate, and silicon emulsions) in an amount within the range of 0.5–5 wt %, a binder (corn starch, polymers, and natural gums), and 0.5–5 wt % of a flowability aid (colloidal silica, and micronized clay). All examples use significant quantities of ammonium sulfate to form a structural granule. Indeed, example 3 of the '674 patent illustrate the adverse storage effects of formulations that do not contain ammonium sulfate.
Cummings U.S. Pat. No. 5,369,100 is directed to a formulation that does not use a binder. Instead, the formulation relies on compaction of a mix containing the technical form of the active an ammonium sulfate. Lubricants (Mg stearate) and flow aids (silica particles) are also added to the formulation as shown in the examples.
Cummings et al. U.S. Pat. No. 6,013,272 teaches the manufacture of water-free phosphoroamido(di)thioate granules without added solvent by heating the extrusion die to a temperature that is sufficient to soften the active solids while controlling the rate at which water is added. Final products are disclosed as having a moisture level of less than 0.5 wt %. It is disclosed in column 5 that small amounts of a vinylpyrrolidone-vinyl acetate copolymer does not adversely affect the process and that the process does not require the use of surfactants or binding agents.
Unfortunately, the phosphoroamido(di)thioates can begin the degradation process under the effects of moisture (hydrolysis) or heat (oxidation). Either degradation mechanism can change the nature and amount of the insecticidally active compound into by-products that are not insecticidally effective. Thus, the moisture content of the final granule should be as low as possible and exposure to heat in processing or storage should be avoided, if possible. It would be beneficial to have an extrusion process for making granules from phosphoroamido(di)thioate solids and other solids adversely affected by heat or moisture without exposing these solids to high temperatures or high levels of residual moisture.